The role of Cu and Ni/Cu substitution effect in austenitic stainless steel

Abstract

Austenitic stainless steels (ASSs) are widely used in various industries due to their excellent corrosion resistance, superior ductility, and high rate of work hardening. However, their low yield strengths and high alloying costs often limit their competitiveness in the steel market, which demands environmentally friendly, high-performance, and cost-effective materials. Recently, there has been interest in Cu-bearing ASSs as a potential solution for high-strength, high-corrosion-resistant structural steels. Cu acts as an austenite stabilizer and can improve the formability of ASSs, while Cu can also enhance mechanical properties by forming fine Cu-rich precipitates in the matrix, making it a promising candidate to replace Ni. Cu However, previous research has mainly focused on the effects of Cu on phase stability or precipitation behavior, with limited studies investigating how Cu achieves these effects in ASSs. Empirical equations have been proposed to describe the effects of Cu on yield strength and ultimate tensile strength, but the influence of Cu on deformation behaviour remains unclear.

Therefore, in the present study, the following three research themes were investigated to clarify the effect of Cu on ASSs and to evaluate the potential of Cu for substituting Ni. 1) Effect of Cu on stacking fault energy (SFE) and deformation behaviour were experimentally and thermodynamically examined in Fe-Cr-Ni ASSs by varying the Cu content. It was revealed that Cu is effective on increasing SFE, but excessive addition of Cu induced the local stress localization and caused earlier fracture of the ASS. 2) Based on the elucidated Cu effect, the actual effects of Ni/Cu substitution and subsequent aging treatments on the tensile properties of the ASSs was investigated, by considering the SFE microstructural evolution. Research indicated that Ni/Cu replacement and aging treatment effectively increased the strength of the steels by precipitation hardening combined with TWIP and TRIP effect, which also contributed to the cost reduction. 3) Alloy compositions were further optimized from the alloys at the second theme to further reduce the amount of Ni and to design the Cu-added low-Ni ASSs. By adding N and Mn based on the alloy composition of theme 2, strength of the steels was significantly increased, while phase stability was also maintained, indicating that newly designed alloy composition in theme 3 is well optimized for Ni reduction and increment of the strength.